14 December 2008

Is there an engineer in the house?

The good people of Portland, Oregon have been busy discussing a new light rail bridge over the Williamette River. The new bridge would carry the proposed Portland - Milwaukie light rail line as well as foot traffic, and is promoted by local transit body TriMet.

There seems to be considerable interest in which bridge option will be chosen. The three initial options were an arch bridge, a cable-stayed bridge (shown top), and a structure described as a "wave frame" (shown bottom) but which is essentially a somewhat dubious cross between a Vierendeel truss and a suspension bridge. Of those, they seem to have whittled the choice down to several variations on the cable-stayed and "wave frame" options.

The somewhat unconventional "wave" option seems popular with the natives, if a poll at BlueOregon is to be believed. The Portland Spaces blog is sufficiently keen that they interview the designer, architect Miguel Rosales.

You might think that Rosales would know his stuff: he worked on the US$115m cable-stayed Leonard P. Zakim Bunker Hill Bridge in Boston, as well as the Liberty Bridge in Greenville, South Carolina, a curved suspension bridge. It may seem somewhat odd, however, that his website doesn't so much as mention the engineers who played key roles in these structures (Christian Menn in Boston - who returns the favour by making no mention of Rosales; and Schlaich Bergermann in Greenville).

Reading the interview with Rosales at Portland Spaces, there's still no mention of an engineer, and I came away with the distinct impression that this is a bridge promoted by an architect who doesn't really understand bridge design. In the comments at the foot of the interview, Rosales says:

"The flowing top chord follows the flow of forces and moment diagram resulting in a slender and elongated structure. The clean design avoids the visual confusion often found in steel trusses with their multiple layers of diagonal and vertical members."

But of course, it doesn't follow the moment diagram at all, which would be cusped above the piers (i.e. "pointy"), in the same manner as a conventional suspension bridge. And it avoids the truss diagonals only at the expense of requiring massive amounts of additional steelwork to provide equivalent stiffness (and it's stiffness that's normally critical on a light-rail bridge).

It's far from clear how the bridge (which spans about 200m) actually works - the very slender depth at midspan provides insufficient moment continuity for it to work as an efficient continous truss, while the weedy vertical members lack the stiffness required for suspension bridge towers. If, as it appears, it's a bizarre hybrid self-anchored suspension bridge relying on Vierendeel action, throughout, it's no surprise that TriMet's cost estimates show it as significantly more expensive than the cable-stayed alternative.

Rosales is keen to promote his design, and suggests that TriMet's engineers have been unreasonable with their cost estimates. From the TriMet website[PDF], it's apparent they have put considerable effort into trying to make the Rosales design work, but still estimate its cost at US$119m, against US$93m for the cable-stay option. That works out at about US$10.5k per square metre of deck for the "wave frame", or about £7,000. That seems about right for an unconventional design like this, but it's easy to suggest it could be more.

4 comments:

Knit MN
said...

Having read the article I am rather confused as to why the team submitted the cable-stayed bridge design as it seems Rosales really doesn't like it, and he says he would have to change it if it were chosen. Has it been submitted as a red herring, I wonder?

All three of the bridge options (arch, cable-stayed and "wave frame") had their visualisations done by the Rosales team (see BlueOregon). Both the arch and cable-stayed images are of fairly clumsy versions of those forms - it would be quiet easy to come up with more elegant versions. The members in the wave-frame illustration, by comparison, have been drawn as slender and pale as possible. Fluting is also shown on the piers of the wave-frame option while the other two options are shown only with flat concrete faces. One possible conclusion is therefore that the images were set up deliberately to bias the viewer's preference.

Please refer to the Portland Spaces Magazine blog in which the engineers working with Rosales + Partners have been identified as Schlaich Bergermann und Partner (SBP), a well known international bridge firm. SBP has completed extensive and comprehensive calculations on the proposed Rosales "wave" bridge and believe the bridge is feasible and structurally sound. TriMet might be able to provide you a copy of the structural report which could be of interest to you. On another topic please note that the Rosales website at www.rosalespartners.com does give credit to all engineers who have collaborated with the firm's principal in the "credits & copyright" section in the home page. Please refer to it in the future.

Good to see that SBP are involved, although it's a shame that Rosales didn't give them clearer credit in the interview. SBP, of course, made their name for structurally innovative engineer-led designs, not for just helping bring an architect's visions to fruition.

I would suggest that "feasible" and "structurally sound" are not the same thing as "a good idea". Pretty much anything can be made to stand up if you throw enough money at it - it's far harder to come up with an elegant idea in the first place, one which makes the structural engineering easy, and the wave bridge is certainly not that.

It's also good to see that the Rosales website does credit the engineers involved; shame it's hidden away under a link nobody will normally bother to follow. I think the website doesn't make Rosales involvement in many of the designs very clear e.g. stating Rosales carried out the "Conceptual, preliminary, and final design" for the Liberty Bridge, a structure very similar to another SBP bridge, but again they are not credited on the relevant project page. Design of structures like this is almost invariably a collaborative process, with the bridge engineer often playing the lead role. You'd get no idea of that from the Rosales website: the subtext is that if you want bridges like these, employ the architect, when that simply isn't a necessity.